ALMA observation of evolving magnetized corona in the radio-quiet changing-state AGN NGC 1566

TL;DR

This study uses ALMA observations to analyze the millimeter variability of the changing-state AGN NGC 1566, revealing a likely synchrotron origin of the mm emission linked to the X-ray corona, advancing understanding of radio-quiet AGN emission mechanisms.

Contribution

First detailed analysis of mm continuum variability in a changing-state AGN, suggesting synchrotron emission from the X-ray corona as the primary source.

Findings

Positive correlation between mm and X-ray flux.

Spectral index consistent with optically thick synchrotron emission.

No clear link between mm/X-ray ratio and Eddington ratio.

Abstract

The origin of compact millimeter (mm) continuum emission from radio-quiet AGNs (RQAGNs) is still not fully understood. Changing-state AGNs (CSAGNs) display rapid and strong variability, which can allow us to investigate the origin of the mm emission. We present here the results of the first study of the mm continuum variability of a CSAGN using archival ALMA band 6 ($\sim 230$ GHz) observations of NGC 1566 obtained in 2014-2023. We find a positive correlation between the mm and X-ray flux with an intrinsic scatter of 0.05 dex ($1\sigma$), suggesting a common origin. The mm spectral index ($\alpha_{\rm mm}$) is found in the range of $0.13\pm0.38$ to $-0.26\pm0.53$, consistent with a compact optically thick synchrotron source. No significant correlation was found between the $\alpha_{\rm mm}$ and the mm flux. The mm/X-ray ratio also shows no clear link to the Eddington ratio but is higher in the low-accretion state. We discuss several scenarios about the origin of the mm emission in NGC 1566. We find that synchrotron emission in the magnetized X-ray corona appears to be the most probable origin of mm emission, confirming that mm emission can be used as a tracer of AGN activity in RQAGNs.